The present invention relates to electrical voltage converter circuits, and more particularly to high efficiency voltage converter circuits that comprise partial voltage switching circuits.
Electrical components are typically optimized to operate at a pre-determined voltage such as 19 volts, 12 volts, 5 volts, 3.3 volts, and 1.2 volts. A wide varieties of electrical components are optimized to operate at a wide varieties of voltages, so that it is often necessary to use electrical voltage converter circuits to convert a voltage source at one voltage to a voltage source at a different voltage. An electrical voltage converter circuit, by definition, is an electrical circuit that draw electrical power from one voltage source while outputting electrical power at a different voltage. The input voltage source can be a direct current (DC) voltage source, an alternative current (AC) voltage source, a DC voltage source with ripples, or any type of voltage sources. The output voltage is typically DC, but it also can be other types of waveforms. When the input voltage source is coupled to the main power source, the electrical voltage converter is called an electrical power converter.
Currently, most of commercial electrical voltage converter circuits are switched-mode voltage converters. FIG. 1(a) shows a simplified symbolic diagram for a prior art switched-mode voltage converter called Buck converter. In this example, the power of this circuit is provided by a voltage source (100) with one terminal at voltage Vi while the other terminal at voltage Vsi, as shown in FIG. 1(a). The Vi terminal is coupled to the drain terminal of a metal-oxide-semiconductor (MOS) transistor (101). The gate voltage of the transistor (101) is Vg, and the source voltage of the transistor (101) is Vp, as shown in FIG. 1(a). The source terminal of the MOS transistor is connected to the cathode of an electrical diode (102); the anode of the electrical diode is connected to the Vsi terminal of the voltage source (100). The source terminal of the MOS transistor (101) is also connected to a terminal of an electromagnetic component (103), which is an inductor in this example. The other terminal of the inductor (103) is connected to one terminal of a capacitor (Co), while the other terminal of Co is connected to output ground at voltage Vs. Vsi and Vi can be the same voltage, and they also can be isolated from each other. In this example, the electromagnetic component (103) and the capacitor (Co) forms an electrical filter (105) that provides an output voltage (Vo) that is the average voltage of Vp. This output voltage (Vo) can be used as a voltage source providing electrical power to other circuits (Ld). A duty cycle control circuit (108) uses a sensing circuit (107) to sense the output voltage (Vo) and control the output voltage value by controlling the duty cycle of the switching signal Vp.
The key to achieve high power efficiency for a switched-mode converter is that the MOS transistor (101) must be either fully on or fully off almost all the time. When the transistor (101) is fully on or fully off, the power consumed by the transistor is very small so that a high percentage of the input power is transferred to the output instead of consumed by the converter circuit. Therefore, the waveform of Vp should switch mostly between Vi and Vsi, such as the example shown in FIG. 1(b). When the source voltage Vp is near Vi or Vsi, the power consumed by the MOS transistor (101) is very small. When Vp is not at Vi or Vsi during transient events, the power consumed by the MOS transistor can be large.
For a Buck converter, at ideal condition, the output voltage (Vo) is related to the waveform of Vp as Vo=D*(Vi−Vsi), where D=(Ton/(Ton+Toff)) is called “duty cycle”, Ton is the time when the MOS transistor is turned on in a period, and Toff is the time when the MOS transistor is turned off in a period, as illustrated in FIG. 1(b). The value of the output voltage (Vo) can be controlled by a duty cycle control circuit (108) which detects the level of the output voltage using a sensing circuit (107) as a feedback to determine the value of the duty cycle. FIG. 1(b) also shows the waveform of Vo, which is nearly a constant in this example.
Switch-mode voltage converters are currently the most successful prior art voltage converters. With proper designs, switch-mode converters can achieve better than 90% power efficiency. However, prior art switch mode voltage converters have many problems. One major problem is the voltage stress on the MOS transistor (101). At ideal condition, the voltage stress on the MOS transistor (101) in FIG. 1(a) is the full voltage (Vi-Vsi) of the input voltage source (100). If the input or output voltage is high, prior art switch-mode voltage converters need to use transistors that can tolerate high voltages. Such high voltage transistors are bulky, expensive, and slow. It is therefore highly desirable to reduce the voltage stress applied on switching circuit components. In addition, as illustrated in FIG. 1(b), when Vp is switching from Vs to Vi, there is a voltage overshoot (121) caused by the electromagnetic component (103); similarly, when Vp is witching from Vi to Vs, there is another voltage overshoot (122). These voltage overshoots (121, 122) cause additional stresses on the MOS transistor (101). The overshoot voltage is proportional to the value of the inductor (103), so that one of prior art solution is to use large electromagnetic components to reduce voltage overshoots. However, large electromagnetic components are heavy, bulky, and expensive. It is therefore highly desirable to reduce voltage overshoots without using large electromagnetic components.
FIG. 1(c) illustrates the waveforms of the electrical current (IL) passing through the inductor (103) and the load current (ILd) of the prior art circuit in FIG. 1(a). The inductor current (IL) swings from Ib to Ip when the MOS transistor (101) is turned on, and swings from Ip back to Ib when the MOS transistor is turned off, as shown in FIG. 1(c). The peak current (Ip) can be significantly higher than the load current (IL), which causes power lost due to parasitic resistance of the electromagnetic component. Prior art solution for this problem is to use a large electromagnetic component that has low parasitic resistance, which further increase the size, weight, and cost of the electromagnetic component. It is therefore highly desirable to reduce this current overshoot without increasing the size of the electromagnetic component. The other solution is to increase the frequency of the switching signal (Vp). However, high voltage transistors are slow so that the switching frequency is limited. It is therefore highly desirable to develop voltage converter circuits that can operate at high frequency.
Besides Buck converters, a wide varieties of switch-mode converters have been developed. FIG. 1(d) shows a simplified symbolic diagram for a prior art switched-mode voltage converter circuit called Buck-Boost converter. The structures of this Buck-Boost converter in FIG. 1(d) are similar to those of the Buck convert in FIG. 1(a) except that the connectors to the inductor (143) and the diode (142) is inverted. For a Buck-Boost converter, at ideal condition, the output voltage Vo=D/(D−1)*(Vi−Vs), where D is the duty cycle. A duty cycle control circuit (148) controls the duty cycle of the switching signal outputs by the MOS transistor (141) to control the value of the output voltage. The voltage stress problem and the current overshoot problem of Buck-Boost converters are worse than those of Buck converters. FIG. 1(e) shows a simplified symbolic diagram for a prior art switched-mode voltage converter circuit called Flyback converter. The electromagnetic component used in this voltage converter is a transformer (153), instead of an inductor. The ground terminal (Vsi) of the input voltage source (100) is separated from the output ground (Vs) connection, as shown in FIG. 1(e). The input voltage (Vi) is coupled to one input terminal of the transformer (153) through an electrical switch (151), while the ground terminal (Vsi) of the voltage source (100) is connected to the other input terminal of the transformer (153). One output terminal of the transformer (153) is connected to the ground (Vs) while the other output terminal is connected to the anode of an electrical diode (152). The cathode terminal of the diode (152) connected to the output (Vo). A duty cycle control circuit (158) controls the output of an electrical switch (151) in order to control the value of the output voltage (Vo).
FIG. 1(f) shows a simplified symbolic block diagram illustrating the general structures of prior art switched-mode voltage converter circuits. An input voltage source (100) provides electrical power to a full voltage switching circuit (161) that outputs a switching signal that switches between the input voltage Vi and input ground voltage Vsi. This switching signal is coupled to an electrical filter (163) that comprises at least one electromagnetic component, which is either a transformer or an inductor. The electrical filter (163) provides an output voltage (Vo) to output loading (Ld), and a duty cycle control circuit (168) controls the value of Vo by controlling the duty cycle of the switching signal generated by the full voltage switching circuit (161). The output voltage (Vo) is therefore controlled to be a waveform that is suitable to be used by the load (Ld) circuit. The output ground (Vs) connection can be the same as the input ground (Vsi), and it also can be isolated from the input ground. These and other prior art switch mode converters are discussed in the book “Fundamental of Power Electronics” authored by Erickson as one of many publications that introduced switched-mode converters. However, all of the prior art switch-mode converters suffers similar voltage stress, voltage overshoot, and current overshoot problems as illustrated in FIG. 1(b, c). It is therefore highly desirable to provide solutions to those problems for switch-mode voltage converter circuits of different configurations.